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The fire broke out at Arizona Pacific Pulp and Paper near 36th Street and University Drive, just south of Sky Harbor International Airport.

The fire was burning paper and cardboard, sending massive plumes of smoke into the air that could be seen around the Valley.

There were at least 25 engines and 10 ladders on scene.

"Because of the heat and the elements, we also have a large contingent on scene to relieve the crews that have been working for quite some time now with heavy gear in very smoky conditions," said Phoenix Fire Capt. Mark Vanacore. "They're probably a little worn out, so we have extra people on the way to relieve these crews and get them some rest and rehydrate."

Firefighters were working against the wind but were able to keep the flames from spreading to nearby structures and vehicles.